Ok, I realize that might be a bit overpowered.... so then I assumed it was just the mastery from gear:
15% + 4,72 * 1,25% = 20.9% hot. Sounds reasonable doesn't it?

Turns out i'm getting a hot that's not even 16% of the direct heals i'm doing..... because aparantly the mastery works like this:
15% * (1+1,25%*4,72) = 15.88%.......

Right now i'm getting roughly 12% haste from gear, which you could translate as a 12% hps gain, while all the mastery on my gear barely adds up to 1%? It's late and maybe i'm completely missing the point, but i'm really starting to doubt my original choice to balance haste and mastery now (Preferring haste, but not being shy about mastery since I end up as disc every now and then).

Can someone explain to me why this mastery is supposedly just as good or even better as haste? Because for me these numbers don't even seem to be in the same ballpark.

The mastery tooltip includes your gear.
The holy mastery is 1.25% per point - you get 8 points base plus whatever is on your gear. So at 12.72 mastery you should be getting 15.9%, which is almost exactly what you're seeing.

The mastery tooltip includes your gear.
The holy mastery is 1.25% per point - you get 8 points base plus whatever is on your gear. So at 12.72 mastery you should be getting 15.9%, which is almost exactly what you're seeing.

Yea, I just realized that myself after waking up this morning. Just my luck that both my formula and the actual one give almost exactly the same value in my current gear ><

Too bad Blizzard fails at rounding though, since 15.9% is 16% rounded in my book, yet my tooltip says 15%. This is what confused me..... That, and trying to do math at midnight after 4,5h of raiding.